I haven't watched that many Wurmwood games so my opinion might change or evolve, but here's what I got for now.

1. Wurmwood is an extremely powerful but also a hard caster. Something like Kara, Emadrak, Vyros 2 are also really powerful casters, but they are a lot more straightforward in their playstyle. With Wurmwood its sometimes hard to even tell if you're playing well or not, since there are so many different lines of play you could take. I would assume that not every Wurmwood player you meet will be proficient with him so you shouldn't despair even if you're pushed into an unfavorable matchup.

2. There are two main things Wurmwood brings to the table as a warlock. ž

a.) He denies LOS, and thus most shooting and charges. This is of course extremely powerful as it more or less prevents you from interacting with him and his army. However there are several way I've seen it dealt with. First of all, you need to be aware that the LOS blocking is a lot harder to maintain once the feat is gone. If you look at it this way, its actually quite similar to a lot of other casters with denial feats. In a lot of cases his feat turn won't be much different from a Kaelyssa, EHaley or maybe Borka 2 feat turn. You simply have to accept that there is a single turn where you can't deal with the opposing army. Sometimes you will have to back off and lose some points on scenario and sometimes you will get alphad on. This however is simply what feats do. If you don't have models that counter it in some way you simply have to deal with it and then counter him next turn with your own strengths.

After the feat turn his LOS blocking consists of his 4 inch forest and 2x 3 inch sentry stone forests. However the sentry stone has to base the forest on himself. That means that it has to either move back after it spawns it, making its forest position for next turn too far back to meaningfully blocks LOS or it has to stay in the forest after it moves forward (probably from the backward move it made last turn) in which case it can be killed. The 4 inch upkeep forest will likely lose relevance once the game moves forward and the sentry stones die or get left too far behind to contribute an unbroken forest wall.

In summary, I would say that you should consider this part of his kit as you would any other denial feat. You will need to position far enough forward to threaten the scenario/models next turn, but far enough backward to not die to the scary things you can't target. It's not an easy situation but not an unfamiliar one.

b) His offensive spellcasting is among the best, I would argue the best in the game. He has a cheap magic missile spell in stranglehold, that he can use to assassinate or clear lone models (possibly for controling beasts/jacks too but I haven't seen it thus far). He has hellmouth to absolutely slaughter infantry, he has an armor debuff, which is great on any caster, he can gather souls to go up to fury 12 which lets him go really crazy with all of the above. However worst of all is that he has a fast arcnode that is extremely difficult to kill because he can port it back to himself. This covers just about everything a spellcaster could ever want offensively and is extremely scary. The main weakness I see with the whole package is that its gated by fury if he doesn't get souls, moving the forest, porting around and a boosted spell are probably impossible for him so keep in mind that he's not going to be able to do everything (just as a taster, if he uses hellmouth and ports Cassius back he doesn't even have enough fury to put down a forest without souls, which he probably won't have on turn two when he wants to do all of that). Other than that don't make hellmouths easy is the best advice I have up till now for people who don't have magic denial. P.S. also assassination if he empties himself out maybe?

3. The game doesn't end when you kill Cassius anymore, but it sort of does. If Cassius is dead Wurmwood can't move and because of that he can't really cast spells efficiently since his most potent spells are range 8 which you can easily measure from his base. If he feated he is basically left without any tools that help his army, its almost like playing against someone with no warlock, except there is an immobile tree a bit past your half of the table that you can kill to win. If you can do an assassination run on Cassius its probably worth to lose a heavy over that (though remember that I'm speculating still).

4. If you have a list with AD or really long threat shooting you get one turn of shooting his army before he can really set his LOS blocking up. This mostly applies if you go second, but even if you go first his positioning might be off if he wants the full forest wall up on the second turn since he couldn't advance all that far forward. If you can kill one or two of his sentry stones at this time you can severly cripple wurmwoods plans. a unit of rangers still has to rely on a couple spikes to do it though as arm 18 with wounds is no slouch to kill at range. Also has prowl, but if he's forced to use it he is less effective next turn when his mannikins try to hit stuff with sprays.

I have some more thoughts but maybe I'll write em after I'm done with watching a couple more tree games.

I'm sorry to say you're off mark on WW. It's true that, due to the size of his toolbox itself, there's a lot of options on the table at any given time, hence it's difficult to make the right decision. Hellmouth itself can be used as a means to achieve any of the three conditions (scenario/attrition/CK). You are also correct in stating that killing Cassius is absolutely worth losing a heavy in most circumstances. Your analysis, however, is flawed in a few ways:

1) Since fertilizer triggers off friendly models now, he can have a 16" forest wall in play on turn 1. This limits the opponent's ability to retaliate for 2 turns. It's also one of the biggest reasons why he's so powerful.

2) The 4" AoE gains importance in the late game simply because there's less stuff on the table. Having the option of blocking LoS for the opponent's last heavy is very strong. Similarly, stranglehold is a potent control spell now that the tree is FURY 7 with eyeless sight, making it effectively COST 3 instead of 4, as was the case in Mk.II.

3) He maxes out at fury 10.

4) There's two main problems with Cassius for me.
One: unpacking all the AD stuff can leave the non-AD warbeasts blocked in. Worse still, you have to carefully place every model to get the breathing space you need while not blocking anyone, and being able to get the tree to where it needs to be. This is not just a problem with my list, since pretty much every Wurmwood player has shifting stones, sentry stones and at least one extra tree.
Two: lack of a DEF debuff. Hellmouth only goes so far when infantry clearing is concerned, mostly due to its high cost (although, again, it did go down from 5 to 4 effectively in the transition from Mk. II). This is why I have 8 boostable sprays, purgation and blessed in my list .

5) You did mention that he's not suited for a long game, and you're right. I'd say the best tactic vs. Wurmwood is to wait him out and position well to avoid unpleasant hellmouths wasting your army and clearing out zones. If you can do that (a counter-timewalk feat could help), the game will become progressively easier as he starts running out of stones, arc nodes and souls. Much like with Haley, you "just" need to preserve your force until about turn 3-4.

Besides learning I apparently need to slav my car, I had too much time on my hands tonight and I had a good hard look at PoM with fresh and less biased eyes. The faction may not be as shit as I thought it was. Also updated Lucant list, not sure about the Foundry but I want to see how it does:

I've actually sold off all my fluff bunny PoM stuff before Mk.III hit. Now I'm stuck with a bunch of unplayable garbage and one dervish.

Like your Lucant list, although double TEP sounds way scarier than TEP + three warjacks. Why that thing actually got buffed in the transition is beyond my understanding. What will you pair it with? During yesterday's tournament I've talked to my CoC opponent a bit and he said that there's no good pair anymore. Then again, he dropped a Lucant list with 9" threat ranges against Wurmwood. TEP was still incredible though.